The present invention relates to a micro-machined thin film gas sensor device, and a method of making and using the same. More specifically, the present invention relates to solid-state sensor arrays for the detection of H2, NH3, and sulfur containing gases.
Hydrogen gas is used in variety of applications ranging from semiconductor thin film processing to rocket fuel in the aerospace industry. The combustible nature of hydrogen however, makes its detection vitally important. A common need in each of these and other similar technologies is the ability to detect and monitor gaseous hydrogen. Hydrogen gas sensors that quickly and reliably detect hydrogen over a wide range of oxygen and moisture concentrations are not currently available, and must be developed in order to facilitate the transition to a hydrogen based energy economy
xe2x80x9cHydrogen will join electricity in the 21st Century as a primary energy carrier in the nation""s sustainable energy future.xe2x80x9d (DOE 1995) This bold statement was made as part of the 1995 Hydrogen Vision and reflects the tremendous potential of hydrogen as an energy system. The abundance and versatility of hydrogen suggests that it can provide solutions to problems encountered with current fossil fuel energy systems, such as declining domestic supplies, air pollution, global warming, and national security.
Significant research and development efforts are currently underway to make the widespread use of hydrogen technically and economically feasible. These efforts are directed toward creating the basic building blocks of a hydrogen economy: production, storage, transport and utilization. An underlying need of each of these building blocks is the ability to detect and quantify the amount of hydrogen gas present. This is not only required for health and safety reasons, but will be required as a means of monitoring hydrogen based processes. For example, if hydrogen were to be introduced as an automobile fuel additive, several sensors would be needed to detect potential hydrogen gas leaks, as well as to monitor and provide feedback to regulate the air/fuel/hydrogen mixture.
Although the safety record of the commercial hydrogen industry has been excellent, it is estimated that undetected leaks were involved in 40% of industrial hydrogen incidents that did occur. Emerging hydrogen based energy systems will require hydrogen sensors that are as ubiquitous as computer chips have become in our factories, homes, and in our cars. This means that the ability to produce large volumes of sensors at a low cost is paramount.
In order to support an effective hydrogen detection and monitoring system, the hydrogen sensor element must fulfill several requirements. It needs to be selective to hydrogen in variety of atmospheres (including the oxygen-rich high-humidity environments found in fuel cells). It must have a good signal to noise ratio and a large dynamic range. Speed of detection is a critical requirement to ensure rapid response to potentially hazardous leaks. Long lifetimes between calibrations are desirable in order to minimize maintenance. Low power consumption is requisite for use in portable instrumentation and personnel monitoring devices. Ultimately, these must all be achieved by a safe sensor element that is affordable to manufacture in large numbers, so that safe design principles are the deciding factor in the number and locations of detection points.
Existing gas sensors are not adequate, either technically and/or from a cost standpoint. Issues such as size, thermal range, and lifetime have proven to be substantial hurdles for current technologies to overcome. Therefore, there is a need to address these issues, with the development of MEMS (Micro-Electro-Mechanical Systems) based solid-state sensor arrays for the detection of, NH3, and sulfur containing gases.
Additionally, hydrogen-containing gases such as CH4, C2H6, acetone, methanol etc. are used in large variety of industrial applications ranging from semiconductor thin film processing to petroleum and polymer manufacturing. The combustible nature of many of these gases as well as the always-increasing need for improved process control makes the detection and monitoring of these gases vitally important. Difficulties with the sensors that are currently used to detect these gases are that they are not chemically specific, and often will have similar response for different gases. In addition, many of these sensors are combustion based and rely on the presence of oxygen. Therefore, a need exists for a sensor with reproducible results specific to individual hydrogen containing gases, able to operate in environments with little to no oxygen present. Furthermore, a need exists for a gas sensor having no moving parts, a response time on the order of seconds, with minimal power consumption, and capable of being used in a hand held portable instrument
About one-half of all the sensors used to measure hazardous gases measure hydrogen. The bulk of these systems utilize as the detector element a Group VIIIB metal element (Ni, Pd, Pt) that is heated to catalytically oxidize the hydrogen, with the resulting change in heat load being the measured parameter for determination of the presence of hydrogen.
Sensors of such xe2x80x9chot wirexe2x80x9d type have cross-sensitivity to other easily oxidized materials, such as alcohols and hydrocarbons. Such easily oxidized materials are common components of gases in a semiconductor-manufacturing environment, and in such application the result is frequent occurrence of false alarms.
Since the current generation of hot wire sensors require an oxidation reaction for operation, such sensors are unable to detect hydrogen when it is present in inert gas streams or environments, which are not of a character to support oxidative reaction. This is a severe deficiency of such hot wire sensors and limits their applicability and utility.
It would be a significant advance in the art to provide a sensor overcoming the aforementioned deficiencies of current hot wire sensors.
Another class of sensors includes metal-insulator semiconductor (MIS) or metal-oxide-semiconductor (MOS) capacitors and field effect transistors, as well as palladium-gated diodes. In general however, these sensors are limited to detecting low concentrations of hydrogen.
Because hydrogen is used in such a wide variety of environments, it is desirable to have a sensor that will be reproducible and specific to hydrogen, even with varying concentration of background gases such as oxygen, water and other contaminants.
It is also desirable to have a solid state sensor that has no moving parts, has a response time on the order of seconds, would operate with minimum power consumption, does not require frequent calibration, and could be used in a hand-held portable instrument.
It therefore is one object of the present invention to provide an improved hydrogen sensor.
It is another object of the invention to provide a hydrogen sensor that senses the presence of hydrogen in a reproducible and hydrogen-specific manner.
It is another object of the invention to provide a hydrogen sensor that senses the presence of hydrogen in a reproducible and hydrogen-specific manner, even with varying concentration of background gases such as oxygen, water and other contaminants.
It is yet another object of the present invention to provide a solid state hydrogen sensor that has no moving parts, has a response time on the order of seconds, operates with minimum power consumption, does not require frequent calibration, has a large dynamic detection range, and can be readily embodied as a hand-held portable instrument.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.
The present invention relates in one aspect to a hydrogen sensor, comprising a hydrogen-interactive thin film sensor element on a micro-hotplate structure.
The hydrogen-interactive thin film sensor element of such sensor may comprise a hydrogen-interactive thin film (i) arranged for exposure to an environment susceptible to the incursion or generation of hydrogen and (ii) exhibiting a detectable change of physical property when the hydrogen-interactive thin film is exposed to hydrogen. Such detectable change of physical property may comprise optical transmissivity, electrical resistivity, electrical conductivity, electrical capacitance, magneto-resistance, photoconductivity, and/or any other detectable property change accompanying the exposure of the thin film sensor element to hydrogen. The hydrogen sensor may further include a detector constructed and arranged to convert the detectable change of physical property to a perceivable output, e.g., a visual output, auditory output, tactile output, and/or auditory output.
In one embodiment of the present invention these sensors couple novel thin films as the active layer with a MEMS structure known as a Micro-Hotplate. This coupling results in a H2 gas sensor that has several unique advantages in terms of speed, sensitivity, stability and amenability to large-scale manufacture. Results indicate that this technology has substantial potential for meeting the sensing requirements of a hydrogen based energy economy.
In another embodiment, the hydrogen-interactive thin film is overlaid by a hydrogen-permeable material protecting the rare earth metal thin film from deleterious interaction with non-hydrogen components of the environment being monitored, such as nitrogen, oxygen, ammonia, hydrocarbons, etc. The protective-over layer may include a metal such as Pd, Pt, Ir, Rh, Ag, Au, Co, and/or alloys thereof. However, these alloys may not be sufficient to prevent the rare earth films and their alloys from oxidizing over long time periods. There are several possible mechanisms for this. Oxygen and other like species can diffuse through the rare earth film or film grain boundaries. If this is the case, a thicker rare earth coating may represent a viable solution. However, this solution may result in decreased sensitivity and responsivity. Another possibility is that the rare earth film is embrittled with repeated hydrogen exposure, and failing with time due to the xcex1 to xcex2 phase transformation of rare earth hydrides that occurs at a hydrogen concentration of xcx9c2%. One solution is to suppress this phase transition in rare earth films such as palladium by alloying the rare earth film with suitable elements, such as silver, titanium, nickel, chromium, aluminum or other species known to those skilled in the art. In this manner, the long-term stability of the underlying coating can be improved.
The micro-hotplate structure in the sensor of the invention may be advantageously constructed and arranged for selectively heating the hydrogen-interactive thin film gas sensor element according to a predetermined time-temperature program, e.g., involving cyclic heating of the hydrogen-interactive thin film gas sensor element by the micro-hotplate structure.
The invention relates in another aspect to a hydrogen sensor device, comprising:
a micro-hotplate structure;
a hydrogen-interactive thin film gas sensor element on the micro-hotplate structure; and
a detector for sensing a detectable change of physical property of the film in exposure to hydrogen and generating a correlative output indicative of hydrogen presence.
A power supply may be provided in such device and may be constructed and arranged for actuating the micro-hotplate structure during and/or subsequent to sensing the detectable change of physical property of the rare earth metal thin film in exposure to hydrogen, and/or for energizing the detector.
A further aspect of the invention relates to a method of fabricating a hydrogen sensor on a substrate, comprising:
constructing on the substrate a micro-hotplate structure; and
forming on the micro-hotplate structure a hydrogen-interactive thin film that in exposure to hydrogen exhibits a detectable change of at least one physical property, and wherein the hydrogen-interactive thin film is arranged to be heated by the micro-hotplate structure.
A still further aspect of the invention relates to a method of detecting hydrogen in an environment, comprising:
providing a hydrogen sensor device comprising a hydrogen-interactive thin film operatively coupled with a micro-hotplate structure for selective heating of the hydrogen-interactive thin film, with the hydrogen-interactive thin film being arranged for exposure to the environment and exhibiting a detectable change of physical property when the hydrogen-interactive thin film is exposed to hydrogen;
exposing the hydrogen-interactive thin film to the environment;
outputting said detectable change of physical property when the presence of hydrogen in the environment is detected; and
selectively heating the hydrogen-interactive thin film by the micro-hotplate structure during and/or subsequent to detection of hydrogen in said environment, to enhance the performance of the hydrogen-interactive thin film for detection of hydrogen.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and claims.